MSVC: force export of API symbols

The public API symbols are compiled in a static library that's linked to core
with --whole-archive when using gcc. As there's no equivalent for this in MSVC
we create a dummy C++ source file that includes every API header file to make
sure that every __declspec(dllexport) is honored.

Change-Id: If0dafedceb52046aecde60b2a2a624df8ab219b4
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>